It’s Bank Holiday weekend and the weather is very windy and unsettled. The rain held off until after lunch, but we could see rain clouds whizzing over our heads all morning. No flying this week, then. Actually, I’ve done no flying outside for the whole of May, due to my visit to Greece and various other non-weather reasons. Let’s hope June turns out to be a lot better.
OK, so I’m stuck indoors and I’ve got a desire to build. As I said last week, I’ve been building a WACO SRE biplane using Autodesk Fusion 360. I finally got some parts out this week, as you can see below.
3 sections of 3d printed WACO wing. Two sections together make a full wing, but you still need to add the curved wing tips. Final span going to be is 32.5cm.
The wing on the left weighs 12g, while the two on the right are both 25g. They’re also the wrong size and the profile is wrong. That’s 3d printing for you. This was my first experiment to see whether to design the wings as a skin, or whether to design a more complicated built-up structure which I could then cover with tissue or mylar. By playing around with the 3d print settings in CURA, I was able to get it to print at 12g, where my first attempts were 25g. That’s a huge difference and is all down to the thickness of the skin. Working out the wing loading, the lighter one comes to 18g/dm2, while the heavier two are 36g/gm2. I was surprised, actually, because even the 12g one felt heavy for what it is, when compared to what I know I could achieve with Depron sheet (about 2g!). There’s also another problem with the 3d prints. Have a look at the profile.
I’m using a 0.2mm layer height, so the printer (an Ultimaker 3) can print more accurate when on the fine settings, but I really don’t like the stepped profile. I could print with the end of wing tip that you see here flat on the printing plate, but then I would lose accuracy along the span. Not that the span accuracy is all that good here anyway. A second problem is that the bed heat has warped the wing spanwise. Sitting it on the flat table top, it’s noticeable that it’s not flat and you can wobble it by pressing the left and right tips. It’s twisted along the span so the root edge isn’t flat on the table. In other words the printer has added it’s own wash-out or wash-in depending on how you look at it. I could live with that, as long as I’m careful about left and right wing halves, but the defects leave a rather wavy trailing edge. I might try printing with a Makerbot, as the software produces much lighter prints, but this is too heavy and inaccurate for me. I’m going to have to look into building the structure up in a more conventional way to reduce the weight.
Talking about structures, I had some problems with the wing profile. The ones you see above are just a Bezier curve that I eyeballed to make a passable aerofoil. The real one is going to be a Clark Y (boring, I know), but I’ve had huge problems with Fusion 360. I just can’t figure this piece of software out, despite trying to learn it for the last 6 months. There are no real instructions, apart from lots of instructional videos which are completely useless. They don’t tell you want you need to know. You can follow their example and it works, then try and make your own design and it doesn’t. I might dump Fusion 360 in the end as it’s driving me crazy.
What I did in the end was to write a piece of software to take the data for a Clark Y from Martin Selig’s fantastic archive of profiles (https://m-selig.ae.illinois.edu/ads/coord_database.html ) and generate the profile as a .obj file that I could load into Fusion 360. Then I tried to rescale it and found that I couldn’t. Why? I don’t get it. Try Googling it and they all say you just right click, open the rescale tool and rescale it. No you can’t, the option is greyed out. It won’t work. This is so annoying because I just want to rescale the cord to be 56mm and I’ve got a wing rib. At this point I think I have to accept that I’m a software developer and CAD just isn’t for me. It’s so much easier for me to write a program in Python to take the profile and generate the rib programmatically using a geometry library that I think I should go down that route, otherwise I’m never going to make anything. I’m aware that there are already software packages like Profili 2, WingFoil out there, but it’s easy enough for me to just write what I need. I’ll even make it free to download and use once it’s finished.
OK, so while the WACRO SRE Fusion 360 project fails, project 2 was to create an autogyro using more conventional (no *!%$& CAD) techniques.
I want something that looks antique, so my sketches came out looking something like an Armstrong Whitworth Siskin biplane from just after WW1. Also under consideration are the Gloster Gladiator (my all time favourite biplane) and a Tiger Moth. I like the open cockpit and the open engine makes an interesting feature. Originally, I had wanted a fully moving rotor head, but the Roto-duo in the latest RCM&E (June 2019) has inspired me to start with a simpler, fixed, two rotor design using conventional elevator and rudder. For speed of construction, I’m building a profile fuselage in Depron, rather like my RS352, which slots together as a cross. That’s a flat nose to tail centre section with top and bottom vertical pieces slotted in to make a plus (+) looking directly at the nose. I’m not quite sure how big it’s going to be yet, but it needs to be easily transportable, so somewhere around 20 to 40 cm in length is what I’m aiming for. Later on today I’m going to get all my micro R/C kit out to see what I’ve got. The size and weight of that is going to dictate the size, but I might be stuck for a micro ESC, we’ll just have to see what turns up. As this is intended to be a 2 day build, and tomorrow is a Bank Holiday, I’ll update later with a post on my progress.
OK, it’s off to the spares box and materials storage… and where did I put my drawing board?
flightpad 